SUPERCONDUCTIVITY IN THE QUATERNARY INTERMETALLIC COMPOUNDS LNNI2B2C

THE attainment of unprecedentedly high transition temperatures (T(c)s) in the copper oxide superconductors illustrates how working with more complex chemical systems allows greater opportunity to balance opposi ng forces within a single chemical compound, leading to a better optim ization of physical properties. For many desired properties, materials with optimal chemical complexity have undoubtedly not yet been found. This appears to be the case for the intermetallic superconductors, wh ose study has languished in recent years, and which almost never show T(c)s above 15 K. These are almost all binary compounds with substitut ion-type additives, or, rarely, true ternary compounds such as LuRh4B4 (T(c) = 11.7 K; refs 1, 2). If, as some argue (refs 3, 4), materials such as A(x)C60 (ref. 5) and Ba0.6K0.4BiO3 (refs 6, 7) are conventiona l electron-phonon superconductors with T(c)s of approximately 30 K, th en the absence of higher T(c)s in intermetallic compounds may mean onl y that more complex materials have not been sufficiently explored. We have recently found superconductivity at 23 K (a T(c) equal to that of the previous intermetallic record holder, Nb3Ge; ref. 9) in the quate rnary intermetallic system yttrium-palladium-boron-carbon8, but we wer e unable to identify the superconducting phase. Here we report superco nductivity at temperatures up to 16.6 K for the single-phase quaternar y intermetallic compounds LnNi2B2C (where Ln stands for Y, Tm, Er, Ho or Lu). The presence of the 3d transition metal nickel, and the layere d crystal structure10 raise intriguing questions about the origin of t he superconductivity, and the relatively high T(c)s of these and the Y -Pd-B-C superconductor suggest that there may yet be more surprises in store.